This invention relates to a thermal conductivity detector for a gas chromatograph.
The thermal conductivity detector is the oldest detector which has been used with a chromatograph. If a filament is set in the flow of a gas streaming out of a column and is kept under a heated condition by applying a constant voltage or a constant current, the temperature of the filament changes in response to a change in the composition of the gas because its thermal conductivity is thereby affected. The thermal conductivity detector is structured so as to detect a change in the electric resistance of such a filament caused by a change in its thermal conductivity by converting the change in the resistance into that of voltage and detecting this voltage change by means of a bridge circuit.
Since the thermal conductivity detector is adapted to detect a change in the composition of a gas or its concentration by detecting a change in temperature, it must be designed such that there would be as little temperature changes as possible due to causes other than the composition or the concentration of the gas. Thus, efforts have been made to eliminate the variations in the source voltage not only by using a heater to control the temperature of the thermal conductivity detector cell inclusive of the filament and surrounding it with a thermal insulator so as to eliminate the environmental temperature effects but also by using a constant current source or a constant voltage source for the filament. It has been known, furthermore, to provide two mutually parallel flow routes within the same cell such that a differential output from the filaments inside these flow routes can be detected.
FIG. 2 shows an example of such a prior art thermal conductivity detector cell 1 comprising a metallic block, say, of a stainless steel material, having two gas flow routes 11 (only one of them being shown in FIG. 2) therethrough. Each gas flow route 11 contains a filament 2, and a carrier gas containing sample components from a column 8 of a gas chromatograph flows through one of the flow routes (the xe2x80x9csample flow routexe2x80x9d) while the other flow route (the xe2x80x9creference flow routexe2x80x9d) is adapted to pass only the carrier gas supplied thereto from another column (not shown) of the same kind and size as the column 8 of the gas chromatograph, serving only to provide a flow resistance. The arrows in FIG. 2 show the direction of the gas flow.
A cylindrically shaped heater block 3 with a heater 4 buried inside is intimately in contact with the detector cell 1. The heater 4 has a heater line sealed inside a metallic cylinder through an electrically insulating heat-resistant material. A thermally sensitive resistor serving as a temperature sensor 5 is provided near its surface, and the electric power to be supplied to the heater 4 is controlled by a temperature controller 6 according to a signal received from the temperature sensor 5.
As the temperature near the surface of the heater 4 is thus adjusted to a specified level, the heater block 3 also comes to have about the same temperature, being made of a metal having a high thermal conductivity. Moreover, the detector cell 1 also comes to be of the specified temperature, and both the detector cell 1 and the heater block 3 (inclusive of the heater 4 and the temperature sensor 5 contained therein) are wrapped by a thermally insulating material (not shown) and are thereby thermally insulated from the environment and protected from the effects of the surrounding temperature or the wind.
The column 8 is inside another thermostatic container referred to as the column oven 9, having its temperature controlled by another temperature controlling device (not shown). The column 8 is connected through a joint 81 to a column connector 7 facing the interior of the column oven 9. The column connector 7 is connected to the detector cell 1 through a metallic pipe 71. The structure described above is the same for both of the flow routes in order to maintain a thermal balance between the column 8 and the detector cell 1.
With a prior art detector structured as described above, the temperature of the heater block 3 may change in an oscillatory manner as the power to the heater 4 is switched on and off in order to control its temperature. Such temperature fluctuations are likely to appear as a noise for the detector. In order to minimize such a noise, it has been a common practice to make the heater block 3 substantially larger than the detector cell 1 in size such that the heater block 3 with a larger heat capacity can substantially absorb the temperature variations and its oscillatory changes can be smoothed out. If the heat capacity of the heater block 3 is thus made larger, however, it takes a longer time to raise its temperature. In other words, when the gas chromatograph is started from its original cooled condition, the wait time (or the so-called stabilization time) required for the temperature of the detector to become sufficiently high such that an analysis can be started becomes too long.
If the thermal capacity of the heater block 3 is reduced carelessly in order to shorten the stabilization time, there appears the danger of oscillatory temperature changes, as explained above. In view of this problem, there have been system improvements such as by introduction of a computer for controlling the temperature. Even if the problem of oscillatory temperature variations of the heater is overcome by such means, however, there still remains another problem related to the effects of heat transmitted from the column oven 9. In other words, since the column connector 7 penetrates into the interior of the column oven 9 and is hence directly exposed to the temperature of the column oven 9, its heat is easily transmitted through the metallic tube 71 to the detector cell 1 and disturbs its thermally stabilized condition. A conventional method of avoiding this phenomenon has been to arrange the metallic tube 71 so as to intimately contact the heater block 3 in the middle such that the heat from the oven 9 will be absorbed by the heater block 3 before reaching the detector cell 1. If the heater block 3 is made smaller, however, it can no longer function as a heat sink to effectively absorb the heat from the oven 9. In other words, the detector cell 1 becomes susceptible once again to the oven temperature.
An attempt has been made, in view of the above, to provide the column connector 7 with another heater block having another heater buried therein and to control its temperature. This method is not satisfactory because an additional component for its temperature control becomes necessary and it affects the overall production cost adversely. In addition, there will be an additional problem of oscillatory temperature changes caused as this additional heater is switched on and off, giving rise to the trouble of an increase in the noise.
It is therefore an object of this invention in view of the problems described above, to provide an improved thermal conductivity detector having a short stabilization time.
It is a particular object of this invention to provide such a thermal conductivity detector capable of reducing the ill-effects of heat from a column oven by reducing the heat capacity of its temperature control system.
A thermal conductivity detector embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising not only a thermal conductivity detector cell disposed so as to be heated by heat transmitted from a heater block but also an additional heater block for heating the column connector through which the detector cell is connected to a column of a chromatograph. The heaters for individually heating these two heater blocks are controlled simultaneously and in synchronism by a single temperature controller according to the temperature of the first heater block for heating the detector cell. With a detector thus structured, the column connector is heated by a separate heater such that its temperature is controlled simultaneously with the detector cell to be kept nearly constant. Thus, even if the a heater block for the detector cell is made smaller and its heat capacity is reduced, the effect of the temperature of the column oven on the detector cell can be minimized and the stabilization time at the start-up of a chromatographic analysis can be made shorter by reducing the heat capacity of the temperature control system for the detector.